There is a growing body of randomized controlled trial (RCT) evidence indicating that mindfulness training interventions may reduce stress and improve stress-related disease outcomes. Yet we know little about the underlying active training mechanisms of mindfulness training. Although it is generally believed that mindfulness training interventions foster a capacity to monitor and accept present moment experience, debate currently focuses on whether it is the capacity to monitor experience moment-by-moment that is critical, or whether it is the capacity to both monitor and non-judgmentally accept experience that drives the salutary effects observed in mindfulness training interventions. This proposal offers a new account and dismantling test of these putative mechanisms of mindfulness training and stress reduction. Specifically, we predict that although learning how to monitor present moment experience will improve behavioral measures of attention, learning how to monitor plus accept present moment experience will reduce stress. These predictions will be tested by recruiting N=135 stressed community adults and randomizing them to either the (1) standard 8-week Mindfulness-Based Stress Reduction (MBSR) program which emphasizes training in both monitoring and accepting present moment experience (Monitor + Accept MBSR condition); (2) an adapted MBSR condition that emphasizes monitoring of one's present moment experience only (Monitor Only MBSR); or (3) No Treatment Control: assessment only. Participants will complete 3-days of daily experience sampling (Ecological Momentary Assessment) immediately before and after the 8-week intervention period to measure attentional control and stress perceptions in daily life. In order to measure psychological and HPA-axis stress reactivity to a controlled stressor, participants will also complete a standardized acute stress challenge task (the Trier Social Stress Test, TSST) immediately following the 3-day post-intervention assessment period. This developmental R21 project provides the first dismantling study of mindfulness meditation training, it utilizes cutting-edge daily experience sampling of real life stress (using EMA) and stress biomarkers (salivary cortisol), and will provide important initial information for designing more effective (and efficient) mindfulness training interventions in at-risk stressed patient populations.